The present invention relates to novel purified and isolated nucleotide sequences encoding mammalian Ca.sup.2+ /calmodulin stimulated phosphodiesterases (CaM-PDEs) and cyclic-GMP-stimulated phosphodiesterases (cGS-PDEs). Also provided are the corresponding recombinant expression products of said nucleotide sequences, immunological reagents specifically reactive therewith, and procedures for identifying compounds which modulate the enzymatic activity of such expression products.
Cyclic nucleotides are known to mediate a wide variety of cellular responses to biological stimuli. The cyclic nucleotide phosphodiesterases (PDEs) catalyze the hydrolysis of 3', 5' cyclic nucleotides, such as cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), to their corresponding 5'-nucleotide monophosphates and are consequently important in the control of cellular concentration of cyclic nucleotides. The PDEs in turn are regulated by transmembrane signals or second messenger ligands such as calcium ion (Ca.sup.2+) or cGMP. The PDEs thus have a central role in regulating the flow of information from extracellular hormones, neurotransmitters, or other signals that use the cyclic nucleotides as messengers.
PDEs are a large and complex group of enzymes. They are widely distributed throughout the cells and tissues of most eukaryotic organisms, but are usually present only in trace amounts. At least five different families of PDEs have been described based on characteristics such as substrate specificity, kinetic properties, cellular regulatory control, size, and in some instances, modulation by selective inhibitors. [Beavo, Adv. in Second Mess. and Prot. Phosph. Res. 22:1-38 (1988)]. The five families include:
______________________________________ I Ca.sup.2+ /calmodulin-stimulated II cGMP-stimulated III cGMP-inhibited IV cAMP-specific V cGMP-specific ______________________________________
Within each family there are multiple forms of closely related PDEs. See Beavo, "Multiple Phosphodiesterase Isozymes Background, Nomenclature and Implications", pp. 3-15; Wang et al., "Calmodulin-Stimulated Cyclic Nucleotide Phosphodiesterases", pp. 19-59; and Manganiello et al., "Cyclic GMP-Stimulated Cyclic Nucleotide Phosphodiesterases" pp. 62-85; all in Cyclic Nucleotide Phosphodiesterases: Structure, Regulation and Drug Action, Beavo, J. and Houslay, M. D., Eds.; John Wiley & Sons, New York (1990).
The Ca.sup.2+ /calmodulin dependent PDEs (CaM-PDEs) are characterized by their responsiveness to intracellular calcium, which leads to a decreased intracellular concentration of cAMP and/or cGMP. A distinctive feature of cGMP'-stimulated phosphodiesterases (cGS-PDEs) is their capacity to be stimulated by cGMP in effecting cAMP hydrolysis.
In vitro studies have shown increased PDE activity in response to Ca.sup.2+ /calmodulin in nearly every mammalian tissue studied, as well as in Drosophila, Dictyostelium, and trypanosomes. The level of CaM-PDE in tissues and cellular and subcellular compartments varies widely. Most cells contain at least a small amount of CaM-PDE activity, with the highest tissue levels being found in the brain, particularly in the synaptic areas. Greenberg et al., Neuropharmacol., 17:737-745 (1978) and Kincaid et al., PNAS (USA), 84:1118-1122 (1987). A decrease in cAMP in astrocytoma cells in response to muscarinic stimulation may be due to calcium dependent increases in CaM-PDE activity. Tanner et al., Mol. Pharmacol., 29:455-460 (1986). Also, CaM-PDE may be an important regulator of cAMP in thyroid tissue. Erneux et al., Mol. Cell. Endocrinol., 43:123-134(1985).
Early studies suggested that there are distinct tissue-specific isozymes of CaM-PDEs. Several members of the CaM-PDE family have now been described, including a 59 kDa isozyme isolated from bovine heart, and 61 and 63 kDa isozymes isolated from bovine brain. LaPorte et al., Biochemistry, 18:2820-2825 (1979); Hansen et al., Proc. Natl. Acad. Sci. USA, 79:2788-2792 (1982); and Sharma et al., J. Biol. Chem., 261:14160-14166 (1986). Possible counterparts to the bovine 59 and 61 kDa isozymes have also been isolated from rat tissues, Hansen et al., J. Biol. Chem., 261:14636-14645 (1986), suggesting that these two isozymes may be expressed in other mammalian species.
In addition to molecular weight criteria, other evidence supports both similarities and differences among the CaM-PDE family of isozymes. For example, the 59 kDa heart isozyme and the 61 kDa brain isozyme CaM-PDEs differ in mobility on SDS-PAGE and elution position on DEAE chromatography, and the 59 kDa isozyme has at least a 10-20 fold higher affinity for calmodulin. Oncomodulin, a fetal/onco calcium binding protein present in very high concentrations in the placenta and transformed cells, also binds to the 59 kDa enzyme with a higher affinity than to the 61 kDa enzyme. However, both the 61 kDa brain and the 59 kDa heart isozymes are recognized by a single monoclonal antibody. This antibody binds to the Ca.sup.2+ /CaM-PDE complex with 100-fold higher affinity than to PDE alone. Hansen et al., 1986, supra. The 59 and 61 kDA isozymes have nearly identical substrate specificities and kinetic constants. Krinks et al., Adv. Cyc. Nucleotide Prot. Phosphorylation Res., 16:31-47 (1984) have suggested, based on peptide mapping experiments, that the heart 59 kDa protein could be a proteolytic form of the brain 61 kDa isozyme.
The 63 kDa bovine brain isozyme differs substantially from the 59 and 61 kDa isozymes. The 63 kDa enzyme is not recognized by the monoclonal antibody which binds to the 59 and 61 kDa enzymes. Hansen et al., 1986, supra. The 63 kDa protein is not phosphorylated in vitro by cAMP-dependent protein kinase, whereas the 61 kDa protein is phosphorylated. Further, only the 63 kDa protein is phosphorylated in vitro by CaM-kinase II. Sharma et al., Proc. Natl. Acad. Sci. (USA), 82:2603-2607 (1985); and Hashimoto et al., J. Biol. Chem., 264:10884-10887 (1989). The 61 and 63 kDa CaM-PDE isozymes from bovine brain do appear, however, to have similar CaM-binding affinities. Peptide maps generated by limited proteolysis with Staphylococcal V8 protease, Sharma et al., J. Biol. Chem., 259:9248 (1984), have suggested that the 61 and 63 kDa proteins have different amino acid sequences.
The cGMP-stimulated PDEs (cGS-PDEs) are proposed to have a noncatalytic, cGMP-specific site that may account for the stimulation of cAMP hydrolysis by cGMP. Stoop et al., J. Biol. Chem., 264:13718 (1989). At physiological cyclic nucleotide concentrations, this enzyme responds to elevated cGMP concentrations with an enhanced hydrolysis of cAMP. Thus, cGS-PDE allows for increases in cGMP concentration to moderate or inhibit cAMP-mediated responses. The primary sequence presented recently in LeTrong et al., Biochemistry, 29:10280 (1990), co-authored by the inventors herein, provides the molecular framework for understanding the regulatory properties and domain substructure of this enzyme and for comparing it with other PDE isozymes that respond to different signals. This publication also notes the cloning of a 2.2 kb bovine adrenal cortex cDNA fragment encoding cGS-PDE. See also, Thompson et al., FASEB J., 5(6):Al592 (Abstract No. 7092) reporting on the cloning of a "Type II PDE" from rat pheochromocytoma cells.
With the discovery of the large number of different PDEs and their critical role in intracellular signalling, efforts have focused on finding agents that selectively activate or inhibit specific PDE isozymes. Agents which affect cellular PDE activity, and thus alter cellular cAMP, can potentially be used to control a broad range of diseases and physiological conditions. Some drugs which raise cAMP levels by inhibiting PDEs are in use, but generally act as broad nonspecific inhibitors and have deleterious side effects on cAMP activity in nontargeted tissues and cell types. Accordingly, agents are needed which are specific for selected PDE isozymes. Selective inhibitors of specific PDE isozymes may be useful as cardiotonic agents, anti-depressants, anti-hypertensives, anti-thrombotics, and as other agents. Screening studies for agonists/antagonists have been complicated, however, because of difficulties in identifying the particular PDE isozyme present in a particular assay preparation. Moreover, all PDEs catalyze the same basic reaction; all have overlapping substrate specificities; and all occur only in trace amounts.
Differentiating among PDEs has been attempted by several different means. The classical enzymological approach of isolating and studying each new isozyme is hampered by current limits of purification techniques and by the inability to accurately assess whether complete resolution of an isozyme has been achieved. A second approach has been to identify isozyme-specific assay conditions which might favor the contribution of one isozyme and minimize that of others. Another approach has been the immunological identification and separation into family groups and/or individual isozymes. There are obvious problems with each of these approaches; for the unambiguous identification and study of a particular isozyme, a large number of distinguishing criteria need to be established, which is often time consuming and in some cases technically quite difficult. As a result, most studies have been done with only partially pure PDE preparations that probably contained more than one isozyme. Moreover, many of the PDEs in most tissues are very susceptible to limited proteolysis and easily form active proteolytic products that may have different kinetic, regulatory, and physiological properties from their parent form.
The development of new and specific PDE-modulatory agents would be greatly facilitated by the ability to isolate large quantities of tissue-specific PDEs by recombinant means. Relatively few PDE genes have been cloned to date and of those cloned, most belong to the cAMP-specific family of phosphodiesterases (cAMP-PDEs). See Davis, "Molecular Genetics of the Cyclic Nucleotide Phosphodiesterases", pp. 227-241 in Cyclic Nucleotide Phosphodiesterases: Structure, Regulation, and Drug Action, Beavo, J. and Houslay, M. D., Eds.; John Wiley & Sons, New York; 1990. See also, e.g., Faure et al., PNAS (USA), 85:8076 (1988)--D. discoideum; Sass et al., PNAS (USA), 83:9303 (1986)--S. cerevisiae, PDE class IV, designated PDE2; Nikawa et al., Mol. Cell. Biol., 7:3629 (1987)--S. cerevisiae, designated PDE1; Wilson et al., Mol. Cell. Biol., 8:505 (1988)--S. cerevisiae, designated SRA5; Chen et al., PNAS (USA), 83:9313 (1986)--D. melanogaster, designated dnc.sup.+ ; Ovchinnikow et al., FEBS, 223:169 (1987)--bovine retina, designated GMP PDE; Davis et al., PNAS (USA), 86:3604 (1989)--rat liver, designated rat dnc-1; Colicelli et al., PNAS (USA), 86:3599 (1989)--rat brain, designated DPD; Swinnen et al., PNAS (USA), 86:5325 (1989)--rat testis, rat PDE1, PDE2, PDE3 and PDE4; and Livi et al., Mol. Cell. Biol., 10:2678 (1990)--human monocyte, designated hPDE1. See also, LeTrong et al., supra and Thompson et al., supra.
Complementation screening has been used to detect and isolate mammalian cDNA clones encoding certain types of PDEs. Colicelli et al., PNAS (USA), 86:3599 (1989), reported the construction of a rat brain cDNA library in an S. cerevisiae expression vector and the isolation therefrom of genes having the capacity to function in yeast to suppress the phenotypic effects of RAS2.sup.va119, a mutant form of the RAS2 gene analogous to an oncogenic mutant of the human HRAS gene. A cDNA so cloned and designated DPD (rat dunce-like phosphodiesterase) has the capacity to complement or "rescue" the loss of growth control associated with an activated RAS2.sup.va119 gene harbored in yeast strain TK161-R2V (A.T.C.C. 74050), as well as the analogous defective growth control phenotype of the yeast mutant 10DAB (A.T.C.C. 74049) which is defective at both yeast PDE gene loci (pde.sup.-1, pde.sup.-2). The gene encodes a high-affinity cAMP specific phosphodiesterase, the amino acid sequence of which is highly homologous to the cAMP-specific phosphodiesterase encoded by the dunce locus of Drosophila melanogaster.
Through the date of filing of parent application Ser. No. 07/688,356, there have been no reports of the cloning and expression of DNA sequences encoding any of the mammalian Ca.sup.2+ /calmodulin stimulated or cGMP-stimulated PDEs (PDE families I and II) and, accordingly, there continues to exist a need in the art for complete nucleotide sequence information for these PDEs.